Laser tracker with enhanced illumination indicators

ABSTRACT

A coordinate measurement device includes: first and second angle measuring devices; a distance meter; a position detector; a first collection of illuminators rotatable about the first axis and fixed with respect to the second axis, the first collection configured to provide a first light selected from among at least two different colors of light in a visible spectrum, the first collection configured to make the first light visible from first and second points along the second axis and external to the device, the first and second points on opposite sides of the device; a second collection of illuminators rotatable about the first and second axes, the second collection configured to provide at least a second light selected from among two different colors of illumination in the visible spectrum; and a processor configured to provide a pattern of illumination for the first and second collections.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication No. 61/592,049 filed Jan. 30, 2012, and U.S. ProvisionalPatent Application No. 61/475,703 filed Apr. 15, 2011, the entirecontents of both of which are hereby incorporated by reference. Thepresent application also claims the benefit of U.S. Design PatentApplication No. 29/413811, filed Feb. 21, 2012, the entire contents ofwhich are hereby incorporated by reference.

BACKGROUND

The present disclosure relates to a coordinate measuring device. One setof coordinate measurement devices belongs to a class of instruments thatmeasure the three-dimensional (3D) coordinates of a point by sending alaser beam to the point. The laser beam may impinge directly on thepoint or on a retroreflector target in contact with the point. In eithercase, the instrument determines the coordinates of the point bymeasuring the distance and the two angles to the target. The distance ismeasured with a distance-measuring device such as an absolute distancemeter or an interferometer. The angles are measured with anangle-measuring device such as an angular encoder. A gimbaledbeam-steering mechanism within the instrument directs the laser beam tothe point of interest.

The laser tracker is a particular type of coordinate-measuring devicethat tracks the retroreflector target with one or more laser beams itemits. Coordinate-measuring devices closely related to the laser trackerare the laser scanner and the total station. The laser scanner steps oneor more laser beams to points on a surface. It picks up light scatteredfrom the surface and from this light determines the distance and twoangles to each point. The total station, which is most often used insurveying applications, may be used to measure the coordinates ofdiffusely scattering or retroreflective targets. Hereinafter, the termlaser tracker is used in a broad sense to include laser scanners andtotal stations.

Ordinarily the laser tracker sends a laser beam to a retroreflectortarget. A common type of retroreflector target is the sphericallymounted retroreflector (SMR), which comprises a cube-cornerretroreflector embedded within a metal sphere. The cube-cornerretroreflector comprises three mutually perpendicular mirrors. Thevertex, which is the common point of intersection of the three mirrors,is located at the center of the sphere. Because of this placement of thecube corner within the sphere, the perpendicular distance from thevertex to any surface on which the SMR rests remains constant, even asthe SMR is rotated. Consequently, the laser tracker can measure the 3Dcoordinates of a surface by following the position of an SMR as it ismoved over the surface. Stating this another way, the laser trackerneeds to measure only three degrees of freedom (one radial distance andtwo angles) to fully characterize the 3D coordinates of a surface.

One type of laser tracker contains only an interferometer (IFM) withoutan absolute distance meter (ADM). If an object blocks the path of thelaser beam from one of these trackers, the IFM loses its distancereference. The operator must then track the retroreflector to a knownlocation to reset to a reference distance before continuing themeasurement. A way around this limitation is to put an ADM in thetracker. The ADM can measure distance in a point-and-shoot manner, asdescribed in more detail below. Some laser trackers contain only an ADMwithout an interferometer. U.S. Pat. No. 7,352,446 ('446) to Bridges etal., the contents of which are herein incorporated by reference,describes a laser tracker having only an ADM (and no IFM) that is ableto accurately scan a moving target. Prior to the '446 patent, absolutedistance meters were too slow to accurately find the position of amoving target.

A gimbal mechanism within the laser tracker may be used to direct alaser beam from the tracker to the SMR. Part of the light retroreflectedby the SMR enters the laser tracker and passes onto a position detector.A control system within the laser tracker can use the position of thelight on the position detector to adjust the rotation angles of themechanical axes of the laser tracker to keep the laser beam centered onthe SMR. In this way, the tracker is able to follow (track) an SMR thatis moved over the surface of an object of interest. The gimbal mechanismused for a laser tracker may be used for a variety of otherapplications. As a simple example, the laser tracker may be used in agimbal steering device having a visible pointer beam but no distancemeter to steer a light beam to series of retroreflector targets andmeasure the angles of each of the targets.

Angle measuring devices such as angular encoders are attached to themechanical axes of the tracker. The one distance measurement and twoangle measurements performed by the laser tracker are sufficient tocompletely specify the three-dimensional location of the SMR.

Several laser trackers are available or have been proposed for measuringsix, rather than the ordinary three, degrees of freedom. Exemplary sixdegree-of-freedom (six-DOF) systems are described by U.S. Pat. No.7,800,758 ('758) to Bridges et al., the contents of which are hereinincorporated by reference, and U.S. Published Patent Application No.2010/0128259 to Bridges et al., the contents of which are hereinincorporated by reference.

Previously, laser trackers have provided LEDs as illuminators forconveying information to operators, the information including when ameasurement was in process, when measurement data was invalid, and thelike. However, in the past, the LEDs were obscured from view except fromthe front of the laser tracker. There is a need for a betterillumination system for laser trackers.

SUMMARY

According to an embodiment of the present invention, a coordinatemeasurement device sends a first beam of light to a remoteretroreflector target, the retroreflector target having a position inspace, the retroreflector target returning a portion of the first beamas a second beam. The measurement device includes: a first motor and asecond motor that together direct the first beam of light to a firstdirection, the first direction determined by a first angle of rotationabout a first axis and a second angle of rotation about a second axis,the first angle of rotation produced by the first motor and the secondangle of rotation produced by the second motor. The device alsoincludes: a first angle measuring device that measures the first angleof rotation and a second angle measuring device that measures the secondangle of rotation; a distance meter that measures a first distance fromthe coordinate measurement device to the retroreflector target based atleast in part on a first portion of the second beam received by a firstoptical detector; and a position detector configured to produce a firstsignal in response to a position of a second portion of the second beamon the position detector. The device further includes: a control systemthat sends a second signal to the first motor and a third signal to thesecond motor, the second signal and the third signal based at least inpart on the first signal, the control system configured to adjust thefirst direction of the first beam to the position in space of theretroreflector target; a first collection of illuminators, the firstcollection rotatable about the first axis and fixed with respect to thesecond axis, the first collection configured to provide a first lightselected from among at least two different colors of light, the at leasttwo colors in a visible spectrum, the first collection configured tomake the first light visible from a first point and a second point, thefirst point and the second point along the second axis and external tothe measurement device, the first point and the second point on oppositesides of the measurement device; a second collection of illuminators,the second collection rotatable about the first axis and the secondaxis, the second collection configured to provide at least a secondlight selected from among two different colors of illumination, the atleast two colors in the visible spectrum; and a processor that providesa three-dimensional coordinate of the retroreflector target, thethree-dimensional coordinate based at least in part on the firstdistance, the first angle of rotation, and the second angle of rotation,the processor further configured to provide a pattern of illuminationfor the first collection of illuminators and the second collection ofilluminators.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, exemplary embodiments are shown whichshould not be construed to be limiting regarding the entire scope of thedisclosure, and wherein the elements are numbered alike in severalFIGURES:

FIG. 1 is a perspective view of a laser tracker system with aretroreflector target in accordance with an embodiment of the presentinvention;

FIG. 2 is a perspective view of a laser tracker system with a six-DOFtarget in accordance with an embodiment of the present invention;

FIG. 3 is a block diagram describing elements of laser tracker opticsand electronics in accordance with an embodiment of the presentinvention;

FIG. 4, which includes FIGS. 4A and 4B, shows two types of prior artafocal beam expanders;

FIG. 5 shows a prior art fiber-optic beam launch;

FIG. 6A-D are schematic figures that show four types of prior artposition detector assemblies;

FIGS. 6E and 6F are schematic figures showing position detectorassemblies according to embodiments of the present invention;

FIG. 7 is a block diagram of electrical and electro-optical elementswithin a prior art ADM;

FIG. 8A and 8B are schematic figures showing fiber-optic elements withina prior art fiber-optic network;

FIG. 8C is a schematic figure showing fiber-optic elements within afiber-optic network in accordance with an embodiment of the presentinvention;

FIG. 9 is an exploded view of a prior art laser tracker;

FIG. 10 is a cross-sectional view of a prior art laser tracker;

FIG. 11 is a block diagram of the computing and communication elementsof a laser tracker in accordance according to an embodiment of thepresent invention;

FIG. 12A is a block diagram of elements in a laser tracker that uses asingle wavelength according to an embodiment of the present invention;

FIG. 12B is a block diagram of elements in a laser tracker that uses asingle wavelength according to an embodiment of the present invention;

FIG. 13 is a block diagram of elements in a laser tracker with six-DOFcapability according to an embodiment of the present invention; and

FIGS. 14A, 14B, and 14C show front, perspective, and perspective views,respectively, of a laser tracker having useful features according toembodiments of the present invention.

DETAILED DESCRIPTION

An exemplary laser tracker system 5 illustrated in FIG. 1 includes alaser tracker 10, a retroreflector target 26, an optional auxiliary unitprocessor 50, and an optional auxiliary computer 60. An exemplarygimbaled beam-steering mechanism 12 of laser tracker 10 comprises azenith carriage 14 mounted on an azimuth base 16 and rotated about anazimuth axis 20. A payload 15 is mounted on the zenith carriage 14 androtated about a zenith axis 18. Zenith axis 18 and azimuth axis 20intersect orthogonally, internally to tracker 10, at gimbal point 22,which is typically the origin for distance measurements. A laser beam 46virtually passes through the gimbal point 22 and is pointed orthogonalto zenith axis 18. In other words, laser beam 46 lies in a planeapproximately perpendicular to the zenith axis 18 and that passesthrough the azimuth axis 20. Outgoing laser beam 46 is pointed in thedesired direction by rotation of payload 15 about zenith axis 18 and byrotation of zenith carriage 14 about azimuth axis 20. A zenith angularencoder, internal to the tracker, is attached to a zenith mechanicalaxis aligned to the zenith axis 18. An azimuth angular encoder, internalto the tracker, is attached to an azimuth mechanical axis aligned to theazimuth axis 20. The zenith and azimuth angular encoders measure thezenith and azimuth angles of rotation to relatively high accuracy.Outgoing laser beam 46 travels to the retroreflector target 26, whichmight be, for example, a spherically mounted retroreflector (SMR) asdescribed above. By measuring the radial distance between gimbal point22 and retroreflector 26, the rotation angle about the zenith axis 18,and the rotation angle about the azimuth axis 20, the position ofretroreflector 26 is found within the spherical coordinate system of thetracker.

Outgoing laser beam 46 may include one or more laser wavelengths, asdescribed hereinafter. For the sake of clarity and simplicity, asteering mechanism of the sort shown in FIG. 1 is assumed in thefollowing discussion. However, other types of steering mechanisms arepossible. For example, it is possible to reflect a laser beam off amirror rotated about the azimuth and zenith axes. The techniquesdescribed herein are applicable, regardless of the type of steeringmechanism.

Magnetic nests 17 may be included on the laser tracker for resetting thelaser tracker to a “home” position for different sized SMRs—for example,1.5, ⅞, and ½ inch SMRs. An on-tracker retroreflector 19 may be used toreset the tracker to a reference distance. In addition, an on-trackermirror, not visible from the view of FIG. 1, may be used in combinationwith the on-tracker retroreflector to enable performance of aself-compensation, as described in U.S. Pat. No. 7,327,446, the contentsof which are incorporated by reference.

FIG. 2 shows an exemplary laser tracker system 7 that is like the lasertracker system 5 of FIG. 1 except that retroreflector target 26 isreplaced with a six-DOF probe 1000. In FIG. 1, other types ofretroreflector targets may be used. For example, a cateyeretroreflector, which is a glass retroreflector in which light focusesto a small spot of light on a reflective rear surface of the glassstructure, is sometimes used.

FIG. 3 is a block diagram showing optical and electrical elements in alaser tracker embodiment. It shows elements of a laser tracker that emittwo wavelengths of light—a first wavelength for an ADM and a secondwavelength for a visible pointer and for tracking. The visible pointerenables the user to see the position of the laser beam spot emitted bythe tracker. The two different wavelengths are combined using afree-space beam splitter. Electrooptic (EO) system 100 includes visiblelight source 110, isolator 115, optional first fiber launch 170,optional interferometer (IFM) 120, beam expander 140, first beamsplitter 145, position detector assembly 150, second beam splitter 155,ADM 160, and second fiber launch 170.

Visible light source 110 may be a laser, superluminescent diode, orother light emitting device. The isolator 115 may be a Faraday isolator,attenuator, or other device capable of reducing the light that reflectsback into the light source. Optional IFM may be configured in a varietyof ways. As a specific example of a possible implementation, the IFM mayinclude a beam splitter 122, a retroreflector 126, quarter waveplates124, 130, and a phase analyzer 128. The visible light source 110 maylaunch the light into free space, the light then traveling in free spacethrough the isolator 115, and optional IFM 120. Alternatively, theisolator 115 may be coupled to the visible light source 110 by a fiberoptic cable. In this case, the light from the isolator may be launchedinto free space through the first fiber-optic launch 170, as discussedherein below with reference to FIG. 5.

Beam expander 140 may be set up using a variety of lens configurations,but two commonly used prior-art configurations are shown in FIGS. 4A,4B. FIG. 4A shows a configuration 140A based on the use of a negativelens 141A and a positive lens 142A. A beam of collimated light 220Aincident on the negative lens 141A emerges from the positive lens 142Aas a larger beam of collimated light 230A. FIG. 4B shows a configuration140B based on the use of two positive lenses 141B, 142B. A beam ofcollimated light 220B incident on a first positive lens 141B emergesfrom a second positive lens 142B as a larger beam of collimated light230B. Of the light leaving the beam expander 140, a small amountreflects off the beam splitters 145, 155 on the way out of the trackerand is lost. That part of the light that passes through the beamsplitter 155 is combined with light from the ADM 160 to form a compositebeam of light 188 that leaves that laser tracker and travels to theretroreflector 90.

In an embodiment, the ADM 160 includes a light source 162, ADMelectronics 164, a fiber network 166, an interconnecting electricalcable 165, and interconnecting optical fibers 168, 169, 184, 186. ADMelectronics send electrical modulation and bias voltages to light source162, which may, for example, be a distributed feedback laser thatoperates at a wavelength of approximately 1550 nm. In an embodiment, thefiber network 166 may be the prior art fiber-optic network 420A shown inFIG. 8A. In this embodiment, light from the light source 162 in FIG. 3travels over the optical fiber 184, which is equivalent to the opticalfiber 432 in FIG. 8A.

The fiber network of FIG. 8A includes a first fiber coupler 430, asecond fiber coupler 436, and low-transmission reflectors 435, 440. Thelight travels through the first fiber coupler 430 and splits between twopaths, the first path through optical fiber 433 to the second fibercoupler 436 and the second path through optical fiber 422 and fiberlength equalizer 423. Fiber length equalizer 423 connects to fiberlength 168 in FIG. 3, which travels to the reference channel of the ADMelectronics 164. The purpose of fiber length equalizer 423 is to matchthe length of optical fibers traversed by light in the reference channelto the length of optical fibers traversed by light in the measurechannel. Matching the fiber lengths in this way reduces ADM errorscaused by changes in the ambient temperature. Such errors may arisebecause the effective optical path length of an optical fiber is equalto the average index of refraction of the optical fiber times the lengthof the fiber. Since the index of refraction of the optical fibersdepends on the temperature of the fiber, a change in the temperature ofthe optical fibers causes changes in the effective optical path lengthsof the measure and reference channels. If the effective optical pathlength of the optical fiber in the measure channel changes relative tothe effective optical path length of the optical fiber in the referencechannel, the result will be an apparent shift in the position of theretroreflector target 90, even if the retroreflector target 90 is keptstationary. To get around this problem, two steps are taken. First, thelength of the fiber in the reference channel is matched, as nearly aspossible, to the length of the fiber in the measure channel. Second, themeasure and reference fibers are routed side by side to the extentpossible to ensure that the optical fibers in the two channels seenearly the same changes in temperature.

The light travels through the second fiber optic coupler 436 and splitsinto two paths, the first path to the low-reflection fiber terminator440 and the second path to optical fiber 438, from which it travels tooptical fiber 186 in FIG. 3. The light on optical fiber 186 travelsthrough to the second fiber launch 170.

In an embodiment, fiber launch 170 is shown in prior art FIG. 5. Thelight from optical fiber 186 of FIG. 3 goes to fiber 172 in FIG. 5. Thefiber launch 170 includes optical fiber 172, ferrule 174, and lens 176.The optical fiber 172 is attached to ferrule 174, which is stablyattached to a structure within the laser tracker 10. If desired, the endof the optical fiber may be polished at an angle to reduce backreflections. The light 250 emerges from the core of the fiber, which maybe a single mode optical fiber with a diameter of between 4 and 12micrometers, depending on the wavelength of the light being used and theparticular type of optical fiber. The light 250 diverges at an angle andintercepts lens 176, which collimates it. The method of launching andreceiving an optical signal through a single optical fiber in an ADMsystem was described in reference to FIG. 3 in patent '758.

Referring to FIG. 3, the beam splitter 155 may be a dichroic beamsplitter, which transmits different wavelengths than it reflects. In anembodiment, the light from the ADM 160 reflects off dichroic beamsplitter 155 and combines with the light from the visible laser 110,which is transmitted through the dichroic beam splitter 155. Thecomposite beam of light 188 travels out of the laser tracker toretroreflector 90 as a first beam, which returns a portion of the lightas a second beam. That portion of the second beam that is at the ADMwavelength reflects off the dichroic beam splitter 155 and returns tothe second fiber launch 170, which couples the light back into theoptical fiber 186.

In an embodiment, the optical fiber 186 corresponds to the optical fiber438 in FIG. 8A. The returning light travels from optical fiber 438through the second fiber coupler 436 and splits between two paths. Afirst path leads to optical fiber 424 that, in an embodiment,corresponds to optical fiber 169 that leads to the measure channel ofthe ADM electronics 164 in FIG. 3. A second path leads to optical fiber433 and then to the first fiber coupler 430. The light leaving the firstfiber coupler 430 splits between two paths, a first path to the opticalfiber 432 and a second path to the low reflectance termination 435. Inan embodiment, optical fiber 432 corresponds to the optical fiber 184,which leads to the light source 162 in FIG. 3. In most cases, the lightsource 162 contains a built-in Faraday isolator that minimizes theamount of light that enters the light source from optical fiber 432.Excessive light fed into a laser in the reverse direction candestabilize the laser.

The light from the fiber network 166 enters ADM electronics 164 throughoptical fibers 168, 169. An embodiment of prior art ADM electronics isshown in FIG. 7. Optical fiber 168 in FIG. 3 corresponds to opticalfiber 3232 in FIG. 7, and optical fiber 169 in FIG. 3 corresponds tooptical fiber 3230 in FIG. 7. Referring now to FIG. 7, ADM electronics3300 includes a frequency reference 3302, a synthesizer 3304, a measuredetector 3306, a reference detector 3308, a measure mixer 3310, areference mixer 3312, conditioning electronics 3314, 3316, 3318, 3320, adivide-by-N prescaler 3324, and an analog-to-digital converter (ADC)3322. The frequency reference, which might be an oven-controlled crystaloscillator (OCXO), for example, sends a reference frequency f_(REF),which might be 10 MHz, for example, to the synthesizer, which generatestwo electrical signals—one signal at a frequency f_(RF) and two signalsat frequency f_(LO). The signal f_(RF) goes to the light source 3102,which corresponds to the light source 162 in FIG. 3. The two signals atfrequency f_(LO) go to the measure mixer 3310 and the reference mixer3312. The light from optical fibers 168, 169 in FIG. 3 appear on fibers3232, 3230 in FIG. 7, respectively, and enter the reference and measurechannels, respectively. Reference detector 3308 and measure detector3306 convert the optical signals into electrical signals. These signalsare conditioned by electrical components 3316, 3314, respectively, andare sent to mixers 3312, 3310, respectively. The mixers produce afrequency f_(IF) equal to the absolute value of f_(LO)−f_(RF). Thesignal f_(RF) may be a relatively high frequency, for example, 2 GHz,while the signal f_(IF) may have a relatively low frequency, forexample, 10 kHz.

The reference frequency f_(REF) is sent to the prescaler 3324, whichdivides the frequency by an integer value. For example, a frequency of10 MHz might be divided by 40 to obtain an output frequency of 250 kHz.In this example, the 10 kHz signals entering the ADC 3322 would besampled at a rate of 250 kHz, thereby producing 25 samples per cycle.The signals from the ADC 3322 are sent to a data processor 3400, whichmight, for example, be one or more digital signal processor (DSP) unitslocated in ADM electronics 164 of FIG. 3.

The method for extracting a distance is based on the calculation ofphase of the ADC signals for the reference and measure channels. Thismethod is described in detail in U.S. Pat. No. 7,701,559 ('559) toBridges et al., the contents of which are herein incorporated byreference. Calculation includes use of equations (1)-(8) of patent '559.In addition, when the ADM first begins to measure a retroreflector, thefrequencies generated by the synthesizer are changed some number oftimes (for example, three times), and the possible ADM distancescalculated in each case. By comparing the possible ADM distances foreach of the selected frequencies, an ambiguity in the ADM measurement isremoved. The equations (1)-(8) of patent '559 combined withsynchronization methods described with respect to FIG. 5 of patent '559and the Kalman filter methods described in patent '559 enable the ADM tomeasure a moving target. In other embodiments, other methods ofobtaining absolute distance measurements, for example, by using pulsedtime-of-flight rather than phase differences, may be used.

The part of the return light beam 190 that passes through the beamsplitter 155 arrives at the beam splitter 145, which sends part of thelight to the beam expander 140 and another part of the light to theposition detector assembly 150. The light emerging from the lasertracker 10 or EO system 100 may be thought of as a first beam and theportion of that light reflecting off the retroreflector 90 or 26 as asecond beam. Portions of the reflected beam are sent to differentfunctional elements of the EO system 100. For example, a first portionmay be sent to a distance meter such as an ADM 160 in FIG. 3. A secondportion may be sent to a position detector assembly 150. In some cases,a third portion may be sent to other functional units such as anoptional interferometer 120. It is important to understand that,although, in the example of FIG. 3, the first portion and the secondportion of the second beam are sent to the distance meter and theposition detector after reflecting off beam splitters 155 and 145,respectively, it would have been possible to transmit, rather thanreflect, the light onto a distance meter or position detector.

Four examples of prior art position detector assemblies 150A-150D areshown in FIGS. 6A-D. FIG. 6A depicts the simplest implementation, withthe position detector assembly including a position sensor 151 mountedon a circuit board 152 that obtains power from and returns signals toelectronics box 350, which may represent electronic processingcapability at any location within the laser tracker 10, auxiliary unit50, or external computer 60. FIG. 6B includes an optical filter 154 thatblocks unwanted optical wavelengths from reaching the position sensor151. The unwanted optical wavelengths may also be blocked, for example,by coating the beam splitter 145 or the surface of the position sensor151 with an appropriate film. FIG. 6C includes a lens 153 that reducesthe size of the beam of light. FIG. 6D includes both an optical filter154 and a lens 153.

FIG. 6E shows a novel position detector assembly that includes anoptical conditioner 149E. Optical conditioner contains a lens 153 andmay also contain optional wavelength filter 154. In addition, itincludes at least one of a diffuser 156 and a spatial filter 157. Asexplained hereinabove, a popular type of retroreflector is thecube-corner retroreflector. One type of cube corner retroreflector ismade of three mirrors, each joined at right angles to the other twomirrors. Lines of intersection at which these three mirrors are joinedmay have a finite thickness in which light is not perfectly reflectedback to the tracker. The lines of finite thickness are diffracted asthey propagate so that upon reaching the position detector they may notappear exactly the same as at the position detector. However, thediffracted light pattern will generally depart from perfect symmetry. Asa result, the light that strikes the position detector 151 may have, forexample, dips or rises in optical power (hot spots) in the vicinity ofthe diffracted lines. Because the uniformity of the light from theretroreflector may vary from retroreflector to retroreflector and alsobecause the distribution of light on the position detector may vary asthe retroreflector is rotated or tilted, it may be advantageous toinclude a diffuser 156 to improve the smoothness of the light thatstrikes the position detector 151. It might be argued that, because anideal position detector should respond to a centroid and an idealdiffuser should spread a spot symmetrically, there should be no effecton the resulting position given by the position detector. However, inpractice the diffuser is observed to improve performance of the positiondetector assembly, probably because the effects of nonlinearities(imperfections) in the position detector 151 and the lens 153. Cubecorner retroreflectors made of glass may also produce non-uniform spotsof light at the position detector 151. Variations in a spot of light ata position detector may be particularly prominent from light reflectedfrom cube corners in six-DOF targets, as may be understood more clearlyfrom commonly assigned U.S. patent application Ser. No. 13/370,339 filedFeb. 10, 2012, and Ser. No. 13/407,983, filed Feb. 29, 2012, thecontents of which are incorporated by reference. In an embodiment, thediffuser 156 is a holographic diffuser. A holographic diffuser providescontrolled, homogeneous light over a specified diffusing angle. In otherembodiments, other types of diffusers such as ground glass or “opal”diffusers are used.

The purpose of the spatial filter 157 of the position detector assembly150E is to block ghost beams that may be the result, for example, ofunwanted reflections off optical surfaces, from striking the positiondetector 151. A spatial filter includes a plate 157 that has anaperture. By placing the spatial filter 157 a distance away from thelens equal approximately to the focal length of the lens, the returninglight 243E passes through the spatial filter when it is near itsnarrowest—at the waist of the beam. Beams that are traveling at adifferent angle, for example, as a result of reflection of an opticalelement strike the spatial filter away from the aperture and are blockedfrom reaching the position detector 151. An example is shown in FIG. 6E,where an unwanted ghost beam 244E reflects off a surface of the beamsplitter 145 and travels to spatial filter 157, where it is blocked.Without the spatial filter, the ghost beam 244E would have interceptedthe position detector 151, thereby causing the position of the beam 243Eon the position detector 151 to be incorrectly determined. Even a weakghost beam may significantly change the position of the centroid on theposition detector 151 if the ghost beam is located a relatively largedistance from the main spot of light.

A retroreflector of the sort discussed here, a cube corner or a cateyeretroreflector, for example, has the property of reflecting a ray oflight that enters the retroreflector in a direction parallel to theincident ray. In addition, the incident and reflected rays aresymmetrically placed about the point of symmetry of the retroreflector.For example, in an open-air cube corner retroreflector, the point ofsymmetry of the retroreflector is the vertex of the cube corner. In aglass cube corner retroreflector, the point of symmetry is also thevertex, but one must consider the bending of the light at the glass-airinterface in this case. In a cateye retroreflector having an index ofrefraction of 2.0, the point of symmetry is the center of the sphere. Ina cateye retroreflector made of two glass hemispheres symmetricallyseated on a common plane, the point of symmetry is a point lying on theplane and at the spherical center of each hemisphere. The main point isthat, for the type of retroreflectors ordinarily used with lasertrackers, the light returned by a retroreflector to the tracker isshifted to the other side of the vertex relative to the incident laserbeam.

This behavior of a retroreflector 90 in FIG. 3 is the basis for thetracking of the retroreflector by the laser tracker. The position sensorhas on its surface an ideal retrace point. The ideal retrace point isthe point at which a laser beam sent to the point of symmetry of aretroreflector (e.g., the vertex of the cube corner retroreflector in anSMR) will return. Usually the retrace point is near the center of theposition sensor. If the laser beam is sent to one side of theretroreflector, it reflects back on the other side and appears off theretrace point on the position sensor. By noting the position of thereturning beam of light on the position sensor, the control system ofthe laser tracker 10 can cause the motors to move the light beam towardthe point of symmetry of the retroreflector.

If the retroreflector is moved transverse to the tracker at a constantvelocity, the light beam at the retroreflector will strike theretroreflector (after transients have settled) a fixed offset distancefrom the point of symmetry of the retroreflector. The laser trackermakes a correction to account for this offset distance at theretroreflector based on scale factor obtained from controlledmeasurements and based on the distance from the light beam on theposition sensor to the ideal retrace point.

As explained hereinabove, the position detector performs two importantfunctions—enabling tracking and correcting measurements to account forthe movement of the retroreflector. The position sensor within theposition detector may be any type of device capable of measuring aposition. For example, the position sensor might be a position sensitivedetector or a photosensitive array. The position sensitive detectormight be lateral effect detector or a quadrant detector, for example.The photosensitive array might be a CMOS or CCD array, for example.

In an embodiment, the return light that does not reflect off beamsplitter 145 passes through beam expander 140, thereby becoming smaller.In another embodiment, the positions of the position detector and thedistance meter are reversed so that the light reflected by the beamsplitter 145 travels to the distance meter and the light transmitted bythe beam splitter travels to the position detector.

The light continues through optional IFM, through the isolator and intothe visible light source 110. At this stage, the optical power should besmall enough so that it does not destabilize the visible light source110.

In an embodiment, the light from visible light source 110 is launchedthrough a beam launch 170 of FIG. 5. The fiber launch may be attached tothe output of light source 110 or a fiber optic output of the isolator115.

In an embodiment, the fiber network 166 of FIG. 3 is prior art fibernetwork 420B of FIG. 8B. Here the optical fibers 184, 186, 168, 169 ofFIG. 3 correspond to optical fibers 443, 444, 424, 422 of FIG. 8B. Thefiber network of FIG. 8B is like the fiber network of FIG. 8A exceptthat the fiber network of FIG. 8B has a single fiber coupler instead oftwo fiber couplers. The advantage of FIG. 8B over FIG. 8A is simplicity;however, FIG. 8B is more likely to have unwanted optical backreflections entering the optical fibers 422 and 424.

In an embodiment, the fiber network 166 of FIG. 3 is fiber network 420Cof FIG. 8C. Here the optical fibers 184, 186, 168, 169 of FIG. 3correspond to optical fibers 447, 455, 423, 424 of FIG. 8C. The fibernetwork 420C includes a first fiber coupler 445 and a second fibercoupler 451. The first fiber coupler 445 is a 2×2 coupler having twoinput ports and two output ports. Couplers of this type are usually madeby placing two fiber cores in close proximity and then drawing thefibers while heated. In this way, evanescent coupling between the fiberscan split off a desired fraction of the light to the adjacent fiber. Thesecond fiber coupler 451 is of the type called a circulator. It hasthree ports, each having the capability of transmitting or receivinglight, but only in the designated direction. For example, the light onoptical fiber 448 enters port 453 and is transported toward port 454 asindicated by the arrow. At port 454, light may be transmitted to opticalfiber 455. Similarly, light traveling on port 455 may enter port 454 andtravel in the direction of the arrow to port 456, where some light maybe transmitted to the optical fiber 424. If only three ports are needed,then the circulator 451 may suffer less losses of optical power than the2×2 coupler. On the other hand, a circulator 451 may be more expensivethan a 2×2 coupler, and it may experience polarization mode dispersion,which can be problematic in some situations.

FIGS. 9 and 10 show exploded and cross sectional views, respectively, ofa prior art laser tracker 2100, which is depicted in FIGS. 2 and 3 ofU.S. Published Patent Application No. 2010/0128259 to Bridges et al.,incorporated by reference. Azimuth assembly 2110 includes post housing2112, azimuth encoder assembly 2120, lower and upper azimuth bearings2114A, 2114B, azimuth motor assembly 2125, azimuth slip ring assembly2130, and azimuth circuit boards 2135.

The purpose of azimuth encoder assembly 2120 is to accurately measurethe angle of rotation of yoke 2142 with respect to the post housing2112. Azimuth encoder assembly 2120 includes encoder disk 2121 andread-head assembly 2122. Encoder disk 2121 is attached to the shaft ofyoke housing 2142, and read head assembly 2122 is attached to postassembly 2110. Read head assembly 2122 comprises a circuit board ontowhich one or more read heads are fastened. Laser light sent from readheads reflect off fine grating lines on encoder disk 2121. Reflectedlight picked up by detectors on encoder read head(s) is processed tofind the angle of the rotating encoder disk in relation to the fixedread heads.

Azimuth motor assembly 2125 includes azimuth motor rotor 2126 andazimuth motor stator 2127. Azimuth motor rotor comprises permanentmagnets attached directly to the shaft of yoke housing 2142. Azimuthmotor stator 2127 comprises field windings that generate a prescribedmagnetic field. This magnetic field interacts with the magnets ofazimuth motor rotor 2126 to produce the desired rotary motion. Azimuthmotor stator 2127 is attached to post frame 2112.

Azimuth circuit boards 2135 represent one or more circuit boards thatprovide electrical functions required by azimuth components such as theencoder and motor. Azimuth slip ring assembly 2130 includes outer part2131 and inner part 2132. In an embodiment, wire bundle 2138 emergesfrom auxiliary unit processor 50. Wire bundle 2138 may carry power tothe tracker or signals to and from the tracker. Some of the wires ofwire bundle 2138 may be directed to connectors on circuit boards. In theexample shown in FIG. 10, wires are routed to azimuth circuit board2135, encoder read head assembly 2122, and azimuth motor assembly 2125.Other wires are routed to inner part 2132 of slip ring assembly 2130.Inner part 2132 is attached to post assembly 2110 and consequentlyremains stationary. Outer part 2131 is attached to yoke assembly 2140and consequently rotates with respect to inner part 2132. Slip ringassembly 2130 is designed to permit low impedance electrical contact asouter part 2131 rotates with respect to the inner part 2132.

Zenith assembly 2140 comprises yoke housing 2142, zenith encoderassembly 2150, left and right zenith bearings 2144A, 2144B, zenith motorassembly 2155, zenith slip ring assembly 2160, and zenith circuit board2165.

The purpose of zenith encoder assembly 2150 is to accurately measure theangle of rotation of payload frame 2172 with respect to yoke housing2142. Zenith encoder assembly 2150 comprises zenith encoder disk 2151and zenith read-head assembly 2152. Encoder disk 2151 is attached topayload housing 2142, and read head assembly 2152 is attached to yokehousing 2142. Zenith read head assembly 2152 comprises a circuit boardonto which one or more read heads are fastened. Laser light sent fromread heads reflect off fine grating lines on encoder disk 2151.Reflected light picked up by detectors on encoder read head(s) isprocessed to find the angle of the rotating encoder disk in relation tothe fixed read heads.

Zenith motor assembly 2155 comprises azimuth motor rotor 2156 andazimuth motor stator 2157. Zenith motor rotor 2156 comprises permanentmagnets attached directly to the shaft of payload frame 2172. Zenithmotor stator 2157 comprises field windings that generate a prescribedmagnetic field. This magnetic field interacts with the rotor magnets toproduce the desired rotary motion. Zenith motor stator 2157 is attachedto yoke frame 2142.

Zenith circuit board 2165 represents one or more circuit boards thatprovide electrical functions required by zenith components such as theencoder and motor. Zenith slip ring assembly 2160 comprises outer part2161 and inner part 2162. Wire bundle 2168 emerges from azimuth outerslip ring 2131 and may carry power or signals. Some of the wires of wirebundle 2168 may be directed to connectors on circuit board. In theexample shown in FIG. 10, wires are routed to zenith circuit board 2165,zenith motor assembly 2150, and encoder read head assembly 2152. Otherwires are routed to inner part 2162 of slip ring assembly 2160. Innerpart 2162 is attached to yoke frame 2142 and consequently rotates inazimuth angle only, but not in zenith angle. Outer part 2161 is attachedto payload frame 2172 and consequently rotates in both zenith andazimuth angles. Slip ring assembly 2160 is designed to permit lowimpedance electrical contact as outer part 2161 rotates with respect tothe inner part 2162. Payload assembly 2170 includes a main opticsassembly 2180 and a secondary optics assembly 2190.

FIG. 11 is a block diagram depicting a dimensional measurementelectronics processing system 1500 that includes a laser trackerelectronics processing system 1510, processing systems of peripheralelements 1582, 1584, 1586, computer 1590, and other networked components1600, represented here as a cloud. Exemplary laser tracker electronicsprocessing system 1510 includes a master processor 1520, payloadfunctions electronics 1530, azimuth encoder electronics 1540, zenithencoder electronics 1550, display and user interface (UI) electronics1560, removable storage hardware 1565, radio frequency identification(RFID) electronics, and an antenna 1572. The payload functionselectronics 1530 includes a number of subfunctions including the six-DOFelectronics 1531, the camera electronics 1532, the ADM electronics 1533,the position detector (PSD) electronics 1534, and the level electronics1535. Most of the subfunctions have at least one processor unit, whichmight be a digital signal processor (DSP) or field programmable gatearray (FPGA), for example. The electronics units 1530, 1540, and 1550are separated as shown because of their location within the lasertracker. In an embodiment, the payload functions 1530 are located in thepayload 2170 of FIGS. 9, 10, while the azimuth encoder electronics 1540is located in the azimuth assembly 2110 and the zenith encoderelectronics 1550 is located in the zenith assembly 2140.

Many types of peripheral devices are possible, but here three suchdevices are shown: a temperature sensor 1582, a six-DOF probe 1584, anda personal digital assistant, 1586, which might be a smart phone, forexample. The laser tracker may communicate with peripheral devices in avariety of means, including wireless communication over the antenna1572, by means of a vision system such as a camera, and by means ofdistance and angular readings of the laser tracker to a cooperativetarget such as the six-DOF probe 1584. Peripheral devices may containprocessors. The six-DOF accessories may include six-DOF probing systems,six-DOF scanners, six-DOF projectors, six-DOF sensors, and six-DOFindicators. The processors in these six-DOF devices may be used inconjunction with processing devices in the laser tracker as well as anexternal computer and cloud processing resources. Generally, when theterm laser tracker processor or measurement device processor is used, itis meant to include possible external computer and cloud support.

In an embodiment, a separate communications bus goes from the masterprocessor 1520 to each of the electronics units 1530, 1540, 1550, 1560,1565, and 1570. Each communications line may have, for example, threeserial lines that include the data line, clock line, and frame line. Theframe line indicates whether or not the electronics unit should payattention to the clock line. If it indicates that attention should begiven, the electronics unit reads the current value of the data line ateach clock signal. The clock-signal may correspond, for example, to arising edge of a clock pulse. In an embodiment, information istransmitted over the data line in the form of a packet. In anembodiment, each packet includes an address, a numeric value, a datamessage, and a checksum. The address indicates where, within theelectronics unit, the data message is to be directed. The location may,for example, correspond to a processor subroutine within the electronicsunit. The numeric value indicates the length of the data message. Thedata message contains data or instructions for the electronics unit tocarry out. The checksum is a numeric value that is used to minimize thechance that errors are transmitted over the communications line.

In an embodiment, the master processor 1520 sends packets of informationover bus 1610 to payload functions electronics 1530, over bus 1611 toazimuth encoder electronics 1540, over bus 1612 to zenith encoderelectronics 1550, over bus 1613 to display and UI electronics 1560, overbus 1614 to removable storage hardware 1565, and over bus 1616 to RFIDand wireless electronics 1570.

In an embodiment, master processor 1520 also sends a synch(synchronization) pulse over the synch bus 1630 to each of theelectronics units at the same time. The synch pulse provides a way ofsynchronizing values collected by the measurement functions of the lasertracker. For example, the azimuth encoder electronics 1540 and thezenith electronics 1550 latch their encoder values as soon as the synchpulse is received. Similarly, the payload functions electronics 1530latch the data collected by the electronics contained within thepayload. The six-DOF, ADM, and position detector all latch data when thesynch pulse is given. In most cases, the camera and inclinometer collectdata at a slower rate than the synch pulse rate but may latch data atmultiples of the synch pulse period.

The azimuth encoder electronics 1540 and zenith encoder electronics 1550are separated from one another and from the payload electronics 1530 bythe slip rings 2130, 2160 shown in FIGS. 9, 10. This is why the buslines 1610, 1611, and 1612 are depicted as separate bus line in FIG. 11.

The laser tracker electronics processing system 1510 may communicatewith an external computer 1590, or it may provide computation, display,and UI functions within the laser tracker. The laser trackercommunicates with computer 1590 over communications link 1606, whichmight be, for example, an Ethernet line or a wireless connection. Thelaser tracker may also communicate with other elements 1600, representedby the cloud, over communications link 1602, which might include one ormore electrical cables, such as Ethernet cables, and one or morewireless connections. An example of an element 1600 is another threedimensional test instrument—for example, an articulated arm CMM, whichmay be relocated by the laser tracker. A communication link 1604 betweenthe computer 1590 and the elements 1600 may be wired (e.g., Ethernet) orwireless. An operator sitting on a remote computer 1590 may make aconnection to the Internet, represented by the cloud 1600, over anEthernet or wireless line, which in turn connects to the masterprocessor 1520 over an Ethernet or wireless line. In this way, a usermay control the action of a remote laser tracker.

Laser trackers today use one visible wavelength (usually red) and oneinfrared wavelength for the ADM. The red wavelength may be provided by afrequency stabilized helium-neon (HeNe) laser suitable for use in aninterferometer and also for use in providing a red pointer beam.Alternatively, the red wavelength may be provided by a diode laser thatserves just as a pointer beam. A disadvantage in using two light sourcesis the extra space and added cost required for the extra light sources,beam splitters, isolators, and other components. Another disadvantage inusing two light sources is that it is difficult to perfectly align thetwo light beams along the entire paths the beams travel. This may resultin a variety of problems including inability to simultaneously obtaingood performance from different subsystems that operate at differentwavelengths. A system that uses a single light source, therebyeliminating these disadvantages, is shown in opto-electronic system 500of FIG. 12A.

FIG. 12A includes a visible light source 110, an isolator 115, a fibernetwork 420, ADM electronics 530, a fiber launch 170, a beam splitter145, and a position detector 150. The visible light source 110 might be,for example, a red or green diode laser or a vertical cavity surfaceemitting laser (VCSEL). The isolator might be a Faraday isolator, anattenuator, or any other device capable of sufficiently reducing theamount of light fed back into the light source. The light from theisolator 115 travels into the fiber network 420, which in an embodimentis the fiber network 420A of FIG. 8A.

FIG. 12B shows an embodiment of an optoelectronic system 400 in which asingle wavelength of light is used but wherein modulation is achieved bymeans of electro-optic modulation of the light rather than by directmodulation of a light source. The optoelectronic system 400 includes avisible light source 110, an isolator 115, an electrooptic modulator410, ADM electronics 475, a fiber network 420, a fiber launch 170, abeam splitter 145, and a position detector 150. The visible light source110 may be, for example, a red or green laser diode. Laser light is sentthrough an isolator 115, which may be a Faraday isolator or anattenuator, for example. The isolator 115 may be fiber coupled at itsinput and output ports. The isolator 115 sends the light to theelectrooptic modulator 410, which modulates the light to a selectedfrequency, which may be up to 10 GHz or higher if desired. An electricalsignal 476 from ADM electronics 475 drives the modulation in theelectrooptic modulator 410. The modulated light from the electroopticmodulator 410 travels to the fiber network 420, which might be the fibernetwork 420A, 420B, 420C, or 420D discussed hereinabove. Some of thelight travels over optical fiber 422 to the reference channel of the ADMelectronics 475. Another portion of the light travels out of thetracker, reflects off retroreflector 90, returns to the tracker, andarrives at the beam splitter 145. A small amount of the light reflectsoff the beam splitter and travels to position detector 150, which hasbeen discussed hereinabove with reference to FIGS. 6A-F. A portion ofthe light passes through the beam splitter 145 into the fiber launch170, through the fiber network 420 into the optical fiber 424, and intothe measure channel of the ADM electronics 475. In general, the system500 of FIG. 12A can be manufactured for less money than system 400 ofFIG. 12B; however, the electro-optic modulator 410 may be able toachieve a higher modulation frequency, which can be advantageous in somesituations.

FIG. 13 shows an embodiment of a locator camera system 950 and anoptoelectronic system 900 in which an orientation camera 910 is combinedwith the optoelectronic functionality of a 3D laser tracker to measuresix degrees of freedom. The optoelectronic system 900 includes a visiblelight source 905, an isolator 910, an optional electrooptic modulator410, ADM electronics 715, a fiber network 420, a fiber launch 170, abeam splitter 145, a position detector 150, a beam splitter 922, and anorientation camera 910. The light from the visible light source isemitted in optical fiber 980 and travels through isolator 910, which mayhave optical fibers coupled on the input and output ports. The light maytravel through the electrooptic modulator 410 modulated by an electricalsignal 716 from the ADM electronics 715. Alternatively, the ADMelectronics 715 may send an electrical signal over cable 717 to modulatethe visible light source 905. Some of the light entering the fibernetwork travels through the fiber length equalizer 423 and the opticalfiber 422 to enter the reference channel of the ADM electronics 715. Anelectrical signal 469 may optionally be applied to the fiber network 420to provide a switching signal to a fiber optic switch within the fibernetwork 420. A part of the light travels from the fiber network to thefiber launch 170, which sends the light on the optical fiber into freespace as light beam 982. A small amount of the light reflects off thebeamsplitter 145 and is lost. A portion of the light passes through thebeam splitter 145, through the beam splitter 922, and travels out of thetracker to six degree-of-freedom (DOF) device 4000. The six-DOF device4000 may be a probe, a scanner, a projector, a sensor, or other device.

On its return path, the light from the six-DOF device 4000 enters theoptoelectronic system 900 and arrives at beamsplitter 922. Part of thelight is reflected off the beamsplitter 922 and enters the orientationcamera 910. The orientation camera 910 records the positions of somemarks placed on the retroreflector target. From these marks, theorientation angle (i.e., three degrees of freedom) of the six-DOF probeis found. The principles of the orientation camera are describedhereinafter in the present application and also in patent ‘758. Aportion of the light at beam splitter 145 travels through thebeamsplitter and is put onto an optical fiber by the fiber launch 170.The light travels to fiber network 420. Part of this light travels tooptical fiber 424, from which it enters the measure channel of the ADMelectronics 715.

The locator camera system 950 includes a camera 960 and one or morelight sources 970. The locator camera system is also shown in FIG. 1,where the cameras are elements 52 and the light sources are elements 54.The camera includes a lens system 962, a photosensitive array 964, and abody 966. One use of the locator camera system 950 is to locateretroreflector targets in the work volume. It does this by flashing thelight source 970, which the camera picks up as a bright spot on thephotosensitive array 964. A second use of the locator camera system 950is establish a coarse orientation of the six-DOF device 4000 based onthe observed location of a reflector spot or LED on the six-DOF device4000. If two or more locator camera systems are available on the lasertracker, the direction to each retroreflector target in the work volumemay be calculated using the principles of triangulation. If a singlelocator camera is located to pick up light reflected along the opticalaxis of the laser tracker, the direction to each retroreflector targetmay be found. If a single camera is located off the optical axis of thelaser tracker, then approximate directions to the retroreflector targetsmay be immediately obtained from the image on the photosensitive array.In this case, a more accurate direction to a target may be found byrotating the mechanical axes of the laser to more than one direction andobserving the change in the spot position on the photosensitive array.

FIGS. 14A, 14B show front and perspective views of an exemplary lasertracker having enhanced design features including narrow field-of-view(FOV) indicator lights 4116, wide FOV illuminated side panel 4140,asymmetric features 4112, 4114, touch sensitive buttons 4130, andretracting handle 4150. FIG. 14C shows a perspective view of anexemplary laser tracker with the retracting handle 4150 in the extendedposition. Also shown in FIGS. 14A-14C are finger slots 4168, posts 4152,side grips 4164A, 4164B, the side grips having side-grip indentations4166, and a recessed grip 4160. An advantage of the retractable handleshown in FIGS. 14A-14C is that the tracker 4100, 4190 can be designedwith a very stiff (thick) zenith carriage 14, while still enabling theuse of a handle and while permitting the size of the tracker to beminimized for shipment in a shipping container. In an embodiment, theposts 4152 provide a frictional constraint that causes the retractablehandle 4150 to remain in its current position, which is either an openposition or a retracted position. The finger slots 4168 are configuredto allow a user to insert fingers on either side of the retractablehandle, thereby making it easier to apply force to move the retractablehandle up or down. The retractable handle may be conveniently used incombination with the recessed grip 4160 to enable a user to make use ofboth hands in moving or positioning the tracker. In an embodiment, theretractable handle is sufficiently stiff to enable the tracker to beturned on its side, for example, for storage in or removal from aninstrument case. The side grips permit a user to place hands on oppositesides of the tracker. The side grips are particularly convenient whentransferring position of the tracker from one user to another user. Theside grips may be made of an elastomeric material to provide improvedgripping. The side grips may contain side-grip indentations to furtherimprove gripping. For example, a first user may support the tracker withthe retractable handle, while a second user grabs the tracker by theside grips. For the purposes of the present application, the term “top”refers to the side of the tracker having the retractable handle 4150,and the term “bottom” refers to the side of the tracker having therecessed grip 4160. These terms top and bottom refer to the mostcommonly used orientations for the laser tracker, although the trackercan be used on its side or even upside down.

The narrow FOV indicator lights 4116 can be identified, as seen in thefront view, from left to right as lights one to six. In an embodiment,the two innermost lights—numbers three and four—are red and green. Thered light is illuminated when a measurement is in process. The greenlight is illuminated with a steady glow when the light beam from thelaser tracker is locked onto the target. The green light is illuminatedand flashing when the laser tracker is not locked onto the target butthe position detector is detecting the light beam. In an embodiment, thenext two innermost lights—numbers two and five—are yellow, and theoutermost lights—numbers one and six—are blue. The yellow and bluelights may be used for a variety of purposes—for example, to providesignals to the operator. The functionality of these lights may be madeaccessible to the user through a software development kit (SDK).

The narrow FOV lights enable the operator to see the LEDs at largedistances—for example, at 80 meters from the tracker. Because of thislarge range and narrow FOV, an observer standing to the side of thelaser tracker may be unable to see the indicator lights. To get aroundthis problem, additional red and green indicator lights are locatedbeneath a diffusely scattering side panel in the illuminated side panel4140. For example, red and green lights may be placed to allow viewingfrom either side and even, at a reduced level, from the front or back ofthe laser tracker.

An important reason for including side panels is to ensure that a useris aware that a laser tracker is in operation, thereby providing warningthat can help prevent disruption of a sensitive measurement that is inprogress. For example, in some cases, a user may walk in front of alaser beam, which is currently pointing at a target, thereby disruptingthe measurement and possibly requiring the measurement to be startedagain (for example, when an interferometer is being used without theassistance of an ADM). The use of illuminated side panels may alsodiscourage people from walking near the tracker or walking near theobjects being measured. This can be beneficial when the tracker or theobjects are located on a bendable floor, for example, a relatively thinconcrete floor.

In an embodiment, the illuminated side panels are configured to be seenalong a point on the second axis 18 (the zenith axis), the point locatedon either side of the tracker. In an embodiment, the illuminated sidepanels are further configured to be seen along a point perpendicular tothe first axis 20 and the second axis 18 and lying to the back side ofthe tracker. In an embodiment, the illuminated side panels are furtherconfigured to be seen from all directions relative to the tracker.

In an embodiment, the illuminated side panels are attached to a firstsupport 4172A and a second support 4172B of the zenith carriage 14. Inan embodiment, each illuminated side panel includes two parts, one partattached to a front side of a zenith-carriage support and the other part(not shown) attached to a back side of the same zenith-carriage support.In an embodiment, the illuminated side panels are covered with adiffusely scattering material.

The illuminators in either the side panel or on the front of the trackermay include multicolor LEDs. This type of LED has the ability to producemore than one color of light. A particular type of multi-color LED is anRGB LED that has the ability to produce red, green, and blue light andin most cases to combine these lights to produce a wide range of colorsin the visible spectrum, and even white.

The illuminators on the side panel may also include a light pipeattached to a light source. Such a light pipe may scatter light alongits length thereby providing a widely diffused light that can be seenover a wide range of angles. The light source in a light pipe may be oneor more LEDs or a different type of light source.

In the future, the costs of various types of display elements areexpected to be reduced. In an embodiment, a display element, which maybe an LED, LCD, LCOS, or pico-projector may be mounted on the front,sides, or back of the laser tracker to provide a wide variety of colorsand messages for the user.

Two modes of operation of a laser tracker are frontsight and backsightmodes. Frontsight mode is the normal mode of operation. Backsight modeis the mode obtained by starting in frontsight mode and then carryingout the following steps: (1) rotate the azimuth angle by 180 degrees and(2) rotate the zenith angle to reverse its sign (a vertical directionupward corresponds to zero degrees), thereby pointing the laser beamback in almost the original direction. In making measurements with lasertrackers, it is often desirable to be able to quickly tell, from adistance, whether the laser tracker is in frontsight or backsight mode.The asymmetric features 4112 and 3914 are flipped upside down inbacksight mode and help the operator tell which mode the laser trackeris in. In addition, in backsight mode, the indicator lights flip belowthe laser tracker output aperture, thereby providing the operator with aclear indicator of whether the laser tracker is in frontsight orbacksight mode.

A few operations are frequently performed on a laser tracker. Forexample, a frequently performed operation is sending the laser trackerto the home position. This is done by causing the laser beam to go to anSMR—typically an SMR placed on one of the three magnetic nests 4120.Because the distance from these “home positions” to the gimbal point 22of the laser tracker is known, performing a home operation provides aconvenient way of resetting the distance reference on the ADM or IFM ina laser tracker. In some cases, the operator may want to quickly performa home operation without returning to a computer to execute the homecommand. The touch sensitive buttons 4130 provide an easy way to dothis. In an embodiment, when the operator touches one of the buttons4130, the laser tracker sends the laser beam to the SMR directly abovethe button. The three magnetic nests 4120 may correspond to differentsize SMRs—for example, SMRs having diameters of 1.5 inches, ⅞ inch, and½ inch. Hence the buttons below the magnetic nests also provide theability to switch among SMRs. For example, an operator could easilyswitch from a 1.5 inch SMR to a ½ inch SMR by placing a ½ inch SMR inthe appropriate magnetic nest and pressing the touch sensitive sensorbeneath it. The touch sensors may be based on the use of capacitivesensors, which are available today for low cost. It is possible to makethe touch sensitive buttons responsive to movements close to the buttonsbefore actual physical contact is made. In other words, it is possibleto use proximity sensors. Besides the examples of the uses of touchsensors given here, it is possible to use touch sensors to issue a widevariety of commands to the laser tracker.

While the invention has been described with reference to exampleembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims. Moreover, the use of the terms first, second, etc. do not denoteany order or importance, but rather the terms first, second, etc. areused to distinguish one element from another. Furthermore, the use ofthe terms a, an, etc. do not denote a limitation of quantity, but ratherdenote the presence of at least one of the referenced item.

1. A coordinate measurement device that sends a first beam of light to aremote retroreflector target, the retroreflector target having aposition in space, the retroreflector target returning a portion of thefirst beam as a second beam, the measurement device comprising: a firstmotor and a second motor configured together to direct the first beam oflight to a first direction, the first direction determined by a firstangle of rotation about a first axis and a second angle of rotationabout a second axis, the first angle of rotation produced by the firstmotor and the second angle of rotation produced by the second motor; afirst angle measuring device configured to measure the first angle ofrotation and a second angle measuring device configured to measure thesecond angle of rotation; a distance meter configured to measure a firstdistance from the coordinate measurement device to the retroreflectortarget based at least in part on a first portion of the second beamreceived by a first optical detector; a position detector configured toproduce a first signal in response to a position of a second portion ofthe second beam on the position detector; a control system configured tosend a second signal to the first motor and a third signal to the secondmotor, the second signal and the third signal based at least in part onthe first signal, the control system configured to adjust the firstdirection of the first beam to the position in space of theretroreflector target; a first collection of illuminators, the firstcollection rotatable about the first axis and fixed with respect to thesecond axis, the first collection configured to provide a first lightselected from among at least two different colors of light, the at leasttwo colors in a visible spectrum, the first collection configured tomake the first light visible from a first point and a second point, thefirst point and the second point along the second axis and external tothe measurement device, the first point and the second point on oppositesides of the measurement device; a second collection of illuminators,the second collection rotatable about the first axis and the secondaxis, the second collection configured to provide at least a secondlight selected from among two different colors of illumination, the atleast two colors in the visible spectrum; and a processor configured toprovide a three-dimensional coordinate of the retroreflector target, thethree-dimensional coordinate based at least in part on the firstdistance, the first angle of rotation, and the second angle of rotation,the processor further configured to provide a pattern of illuminationfor the first collection of illuminators and the second collection ofilluminators.
 2. The coordinate measurement device of claim 1, whereinthe first collection is further configured to make the first lightvisible at a third point on a line perpendicular to the first axis andthe second axis, the third point external to the measurement device andon a side of the measurement device opposite the first beam, the firstbeam not coincident with the first axis.
 3. The coordinate measurementdevice of claim 2, wherein the first collection is further configured tomake the first light visible from any direction external to themeasurement device.
 4. The coordinate measurement device of claim 1,wherein some of the first collection of illuminators are light emittingdiodes.
 5. The coordinate measurement device of claim 1, wherein a firstilluminator from the first collection of illuminators emits the samecolor light as a second illuminator from the second collection ofilluminators.
 6. The coordinate measurement device of claim 5, whereinthe light emitted by the first illuminator is synchronized with lightemitted by the second illuminator.
 7. The coordinate measurement deviceof claim 1, wherein the illuminators from the first collection ofilluminators are covered by a diffusing material.
 8. The coordinatemeasurement device of claim 1, wherein the first axis is an azimuth axisand the second axis is a zenith axis.
 9. The coordinate measurementdevice of claim 8, wherein the first collection of illuminators isfixedly attached to a zenith carriage and the second collection ofilluminators is fixedly attached to a payload, the zenith carriageconfigured to rotate about the first axis and the payload configured torotate about the first axis and the second axis.
 10. The coordinatemeasurement device of claim 9, wherein the zenith carriage includes afirst support and a second support, the payload attached to the zenithcarriage by the first support and the second support, the zenithcarriage having a front side and a back side, the front side being in adirection perpendicular to the first axis and the second axis, the backside being a direction opposite the front side.
 11. The coordinatemeasurement device of claim 10, wherein the first collection ofilluminators includes a first part and a second part, the first partattached to the first support and the second part attached to the secondsupport.
 12. The coordinate measurement device of claim 10, wherein thefirst collection of illuminators includes a first segment, a secondsegment, a third segment, and a fourth segment, the first segmentpositioned on the front side of the first support, the second segmentpositioned on the back side of the first support, the third segmentpositioned on the front side of the second support, and the fourthsegment positioned on the back side of the second support.
 13. Thecoordinate measurement device of claim 1, wherein at least oneilluminator is a multi-color LED component, the at least one illuminatorselected from the group consisting of the first collection ofilluminators and the second collection of illuminators.
 14. Thecoordinate measurement device of claim 13, wherein the at least oneilluminator is configured to produce colors that include red, green, andblue.
 15. The coordinate measurement device of claim 14, wherein the atleast one illuminator is configured to produce combinations of thecolors red, green, and blue.
 16. The coordinate measurement device ofclaim 15, wherein the at least one illuminator is configured to producea combination of the colors red, green, and blue that includes white.17. The coordinate measurement device of claim 1, wherein an illuminatorincludes a light pipe originating from one or more light sources, theilluminator selected from the group consisting of the first collectionof illuminators and the second collection of illuminators, the lightpipe further configured to emit a third light along a length of thelight pipe, the third light derived from the one or more light sources.18. The coordinate measurement device of claim 17, wherein the one ormore light sources includes an LED.